DC to DC converters are frequently employed to convert relatively low voltage DC sources into high voltage DC sources. The high voltage DC sources are then suitable for application to a DC load, such as electrodes of an electron tube or other electrical devices.
Referring to FIG. 3, one such DC to DC converter 10 includes a first rectifying and filtering circuit 11, a protecting circuit 13, a transformer 14, a second rectifying and filtering circuit 12, a pulse width modulation (PWM) circuit 15, a rectifying diode 16, and a transistor 19. The transistor 19 is a p-channel metal-oxide-semiconductor field-effect transistor (P-MOSFET).
The PWM circuit 15 includes a voltage input 151 configured to receive an operation voltage and a pulse output 152 configured to provide a pulse signal to a gate electrode of the transistor 19.
The first rectifying and filtering circuit 11 includes two inputs 111, 112 configured to receive an external alternating current (AC) voltage such as a 220V AC voltage, a full-bridge rectifying circuit 110 configured to convert the 220V AC voltage to a first direct current (DC) voltage, a filter capacitor 114 configured to stabilize the first DC voltage, and a first output 113 configured to provide the first DC voltage to the transformer 14. Two inputs of the full-bridge rectifying circuit 110 serve as the two inputs 111, 112. A positive output of the full-bridge rectifying circuit 110 serves as the first output 113. A negative output of the full-bridge rectifying circuit 110 is grounded. The filter capacitor 114 is connected between the first output 113 and ground.
The transformer 14 includes a primary winding 141, an assistant winding 142, and a secondary winding 143. The first output 113 of the first rectifying and filtering circuit 11 is connected to ground via the primary winding 141, a drain electrode and a source electrode of the transistor 19, and a resistor 190 in series. The gate electrode of the transistor 19 is connected to the pulse output 152 of the PWM circuit 15. The protecting circuit 13 is connected in parallel with the primary winding 141.
One terminal of the assistant winding 142 is connected to ground. The other terminal of the assistant winding 142 is connected to the voltage input 151 of the PWM circuit 15 via the anode and the cathode of the rectifying diode 16 in serials.
The second rectifying and filtering circuit 12 includes a second rectifying diode 124. The secondary winding 143 is coupled to a second output 163 via the second rectifying and filtering circuit 12 for providing a second DC voltage to a load circuit (not shown) through the second output 163.
The external AC voltage is provided to the two inputs 111, 112 of the first rectifying and filtering circuit 11 and is transformed into the first DC voltage by the first rectifying and filtering circuit 11. Then the first DC voltage is provided to the primary winding 141. The assistant winding 142 induces the primary winding 141, generates an operation voltage, and provides the operation voltage to the voltage input 151 of the PWM circuit 15 via the rectifying diode 16. Thus the PWM circuit 15 generates the pulse signal for switching on or switching off the transistor 19. When the transistor 19 is switched on, a first current path is formed sequentially through the first output 113, the primary winding 141, the transistor 19, and the resistor 190. A first current I is formed when the first DC voltage provided to the first output 113 is connected to ground via the first current path. The first current I flowing through the first current path linearly increases until electromagnetic induction generated in the primary winding 141 reaches a predetermined maximum threshold.
When the transistor 19 is switched off, the energy stored in the primary winding 141 of the transformer 140 transfers to the secondary winding 143. Thus an AC voltage across the secondary winding 143 is generated. The second rectifying and filtering circuit 12 transforms the AC voltage into the second DC voltage and provides the second DC voltage to the load circuit.
However, because the second DC voltage is generated by second rectifying and filtering circuit 12, a current always flows through the second rectifying diode 124. Since a resistance of the second diode 124 is substantially large, a power consumption of the second diode 124 is correspondingly large which results in a low transform efficiency of the DC to DC converter 10.
It is desired to provide a new DC to DC converter which can overcome the above-described deficiency